A common form of pointing device for use with computers and the like is referred to as a “mouse”. The computer user moves the device over a surface to move a cursor on the computer screen. The amount and direction of motion of the mouse are sensed by the device and determine the distance and direction in which the cursor moves on the screen. Inexpensive mice based on a ball that rolls over the surface have been used for some time. The ball turns two cylinders that sense the distance and direction of motion. Unfortunately, the ball picks up grease and other dirt from the surface and transfers this material to the cylinders. The resulting coating on the cylinders interferes with the motion of the cylinders, and hence, the devices must be cleaned periodically. The cleaning operation is awkward and time consuming. In addition, the numerous mechanical assemblies included in the device increase the cost of assembly.
Mice based on optical sensing avoid this problem. Originally, such mice had to be moved over a special pad that had grid lines that were sensed by the device. The need to use this special pad made these devices less attractive than the mechanical mice discussed above. More recently, optical mice that do not require such pads have been developed. These mice include a light source that illuminates the surface under the mouse at a shallow angle, which accentuates the structural details of the surface. An image sensor in the mouse records an image of the illuminated surface periodically. By comparing two successive images, the displacement of the mouse between the times at which the images were taken can be determined.
Unfortunately, this type of optical mouse does not function properly on a glass-covered surface such as the glass tops used on many desks or other work surfaces. The glass covers are used to protect the underlying surface. The top surface of the glass is too smooth to provide an image that has sufficient structure to measure the displacement of the mouse. While the surface under the mouse may have the required structure, the imaging sensor and optics in the mouse do not provide an in-focus image of the underlying surface. Hence, traditional optical mice have not been useable on many glass-covered desktops.
One solution to the out-of-focus image problem discussed above is described in co-pending U.S. patent application Ser. No. 10/404,252. In that patent, the mouse is provided with a spacer that alters the focal plane of the imaging system in the mouse. When a user wishes to employ the mouse on a glass surface, the user changes the spacer manually to alter the distance from the focal plane of the optical system to the image sensor. The mouse can then operate using the surface under the glass plate.
While this solution represents a significant improvement over conventional optical mice, it still has a number of problems. First, the user must adjust the distance between the sensor and the underlying glass plate by an amount that compensates for the thickness of the glass plate. This places the burden of adapting the mouse on the user who may not have the required level of expertise.
Second, this solution only works if the surface below the glass surface has sufficient detail. If the underlying surface is too smooth, even this solution will not provide adequate structure to allow the mouse to function. In this regard, it should be noted that the intervening glass plate decreases the signal-to-noise ratio in the image due to reflections at the glass boundaries. Hence, the minimum degree of structure in the underlying surface must be greater than that needed by an optical mouse that is not operating through a glass plate.